The requirement for real time information as to the indoor location of products of value, such as PC's, hospital equipment etc which are routinely subject to relocation has led to proposals such as described in
US 2005/0232081
US 2005/128099
US 2006/013070
etc.
Also in GB 2305074 is described a data signal processor that uses Goertzel's algorithm to identify carrier modulation and data transmission system used by a tag to communicate data to a reader.